Clones with reduced chromosomal heterogeneity will be derived from various malignant mouse cell lines and used to make somatic cell hybrids with nonmalignant or other malignant cells. Transplantability of the hybrids will be determined and corrected with karyotype to test the hypothesis that (1) Complementation can occur so that some malignant x malignant crosses will be non-malignant, and (2) Specific chromosome loss from a hybrid can lead to return of transplantability. A heat-resistant cell line has been derived from a malignant cell line. Its transplantability will be determined and any change correlated with karyotypic changes. Studies of histocompatibility loss variants will be carried out in an attempt to assess the role of gross chromosomal changes in this phenotypic variation.